The present invention relates to a nuclear power station primary pump.
A primary pump in a nuclear power station comprises a drive shaft for rotating a wheel, and above the wheel and concentrically about the shaft, a heat exchanger forming a thermal barrier, a thermal barrier cover having a peripheral skirt provided at its base with a circular end wall extending beneath the heat exchanger, and a pump diffuser having a peripheral skirt concentric with the peripheral skirt of the thermal barrier cover and provided at its base with a rim extending beneath the circular end wall of the body.
This type of primary pump also has a ring known as a xe2x80x9clabyrinthxe2x80x9d ring placed between the heat exchanger and the shaft, and a body having the heat exchanger and the peripheral skirt of the thermal barrier cover fixed thereto. This body is connected to the thermal barrier cover by link elements.
Until now, the circular end wall of the thermal barrier cover has extended immediately beneath the bottom portion of the heat exchanger and has co-operated with the rim of the diffuser to define a bottom chamber for cold water.
In addition, the rim of the diffuser has had a central bore for passing the shaft and has been provided with a succession of circumferential and superposed rings forming a lip seal of the labyrinth type.
That disposition presents drawbacks.
The diffuser rim carrying the lip seal of the labyrinth type is placed in the transition zone between the hot water of the primary circuit which is at a temperature of about 300xc2x0 C. and the cooled zone of the pump which is at a temperature of about 40xc2x0 C.
The diffuser rim, and consequently also the labyrinth type seal, are both subjected to a large temperature gradient which encourages thermal fatigue cracks to form on the lips of the seal.
An object of the invention is to avoid those drawbacks by proposing a nuclear power station primary pump in which the distribution of temperatures at the labyrinth type lip seal between the hot water of the primary circuit and the cooled zone is modified in such a way as to reduce crack formation in the seal.
The invention thus provides a nuclear power station primary pump of the type comprising firstly a drive shaft for rotating a wheel and, above said wheel and concentrically about said shaft, a heat exchanger forming a thermal barrier, a thermal barrier cover comprising a peripheral skirt provided at its base with a circular end wall extending beneath the heat exchanger, and a diffuser comprising a peripheral skirt provided at its base with a rim extending beneath the circular end wall of the thermal barrier cover, and secondly a body having fixed thereto the heat exchanger and the peripheral skirt of the thermal barrier cover, wherein the circular end wall of the thermal barrier cover co-operates with the base of the heat exchanger to define a bottom chamber for cold water and includes a central bore provided with a succession of circumferential and superposed rings forming a lip seal of the labyrinth type, and wherein the circular end wall cooperates both with the peripheral skirt of the thermal barrier cover and with the rim of the peripheral skirt of the diffuser to define clearance forming a space for maintaining stagnant cooling fluid.
According to other characteristics of the invention:
the circular end wall has vertical ribs subdividing the bottom chamber into a plurality of compartments;
the free end of each rib includes facing, and the facings together form a circular sector for centering a ring placed between the shaft and the heat exchanger;
a sealing ring is interposed between the top edge of the peripheral skirt of the thermal barrier cover and the end wall of the body; and
the circular end wall of the thermal barrier cover includes a zone of reduced thickness between the bore and the peripheral skirt.